In the next generation mobile communication, a high-speed data transmission is demanded. In order to perform a high-speed data transmission, a wide frequency band is necessary. However, in a radio communication channel, frequency selective fading including a number of delayed waves (multi-paths) occurs. When performing a wideband single-carrier transmission in such a communication environment, the sampling intervals of received signals become shorter, and therefore, the delayed wave in the channel causes inter-symbol interference, resulting in considerable deterioration in the transmission performances. There is a technique to suppress the inter-symbol interference by time domain signal processing, however, there is such a problem that the circuit scale of a receiver becomes very large.
Hence, a multi-carrier transmission attracts attention, such as orthogonal frequency division multiplexing (OFDM) in which a large amount of data is transmitted in parallel in a frequency domain using a number of narrowband subcarriers arranged at orthogonal frequency intervals and multi-carrier code division multiple access (MC-CDMA).
In the multi-carrier transmission, the periodicity of the OFDM symbol can be maintained by adding a waveform at the rear of an OFDM symbol interval to the front as a guard interval (GI). Hence, it is possible to perform signal processing that does not depend on the number of paths while each subcarrier maintains orthogonality between subcarriers without being affected by the multi-path even in the frequency selective fading environment. Hence, in LTE (Long Term Evolution) that is conformal with the standard of 3GPP (3rd Generation Partnership Project), OFDM is employed in a downlink (transmission from a base station to a mobile terminal). Further, OFDM is also used in terrestrial digital broadcasting, wireless LAN (Local Area Network), etc.
On the other hand, in an uplink (transmission from a mobile terminal to a base station) in mobile communication, it is difficult to use the OFDM transmission with a high PAPR (Peak to Average Power Ratio) because of the problem of nonlinearity of a transmit power amplifier of a mobile terminal and it is desired to use a single-carrier transmission. Thus, in the uplink of LTE, a single-carrier transmission called SC-FDMA (Single Carrier Frequency Division Multiple Access (also called DFT-S-OFDM)) is employed. In SC-FDMA, it is possible to achieve excellent transmission performances with a frequency diversity effect obtained while suppressing inter-symbol interference by adding GI to a single-carrier transmission and using equalization to compensate for the distortion of a signal by one-time multiplication in the frequency domain (frequency domain equalization) based on the same concept as that of OFDM. As described above, in the present situation of the recent mobile communication, different transmission schemes are employed in the uplink and downlink.
However, in the single-carrier transmission using frequency domain equalization, the inter-symbol interference cannot be suppressed completely, and therefore, there is such a problem that the transmission performances are somewhat deteriorated compared to those in the multi-carrier transmission (for example, OFDM). Further, in the single-carrier transmission, adaptive modulation on the frequency domain cannot be performed, and therefore, the throughput (spectral efficiency) is lowered compared to the multi-carrier transmission.
Because of such a problem, Patent Document 1 proposes the transmitter, receiver, mobile communication system, and transmission control method capable of switching between the single-carrier type radio access system and the multi-carrier type radio access system. In Patent Document 1, a switching part to switch the transmission schemes is provided in the transmitter. The switching part selects the single-carrier transmission when the required transmit power is high and the multi-carrier transmission with a high PAPR cannot be performed. On the other hand, the transmitter which has the low transmit power and for which PAPR is not problematic selects the multi-carrier transmission. This can improve the throughput of the entire cell.
As described above, it has also been proposed to cause mobile terminals that use different transmission schemes to exist in the uplink as well as using different transmission schemes in the uplink and downlink and the proposal is now actively discussed in 3GPP (Non-patent Document 1).    Patent Document 1: Japanese Unexamined Patent Publication No. 2007-151059    Non-patent Document 1: R1-082575, NTT DoCoMo